(1) Field of the Invention
The present invention relates to a method for using coal to reduce iron ore efficiently in a fluidized bed to obtain a reduced iron.
(2) Description of the Prior Art
Iron and steel production mainly depends on the method such as a blast furnace-converter. However, a direct reduction which obtains a reduced iron by a shaft furnace or a rotary kiln has been put to practical use. (1) in the blast furnace-converter process, iron ore powder is pre-processed into an agglomerate (pellet, sinter, briquet, etc.) or lump iron ore is necessary to be used. The same is true for the direct reduction. However, if the iron ore powder can be reduced while using it as a raw material, process for forming into an agglomerate is cut down. (2) In the iron making process using the blast furnace, a large quantity of coke is used and therefore a coke oven has to be provided. Therefore, large-scaled equipment and a material storing yard and the like are required. Selection of raw coal is also important to produce a high quality coke which is used for blast furnace. (3) On the other hand, a direct reduction using fluidized bed has been studied and put into practical use. In the direct reduction, only a method of using reducing gas which is obtained by reforming natural gas has been commercially installed. A method of gasifying coal to obtain a reducing gas to use in the direct reduction of iron ore as a reducing agent can be established in a technical sense but has not been carried out as that is expensive.
The most significant problem of the fluidized bed reduction is the so called "sticking" problem in which iron ore particles aggromelete into larger granules and fluidization can not continue. To prevent this phenomena, operation temperature should be held lower than 800.degree. C. at the sacrifice of reduction velocity. The following have been proposed as the other means to prevent the sticking: (1) coarse iron powder is used, (2) the interior of a fluidized bed reactor is mechanically agitated by means of an agitator or the like, (3) coke powder or the like is mixed into the fluidized bed reactor, and (4) an iron ore powder is coated with a nonferrous oxide such as MgO or CaO. These methods have been found to be considerably effective in an experimental stage. However, employment of these methods for practical use poses problems that in method (1), cost for adjustment of grain size is increased and a reaction velocity is lowered; in method (2), an impeller becomes worn due to the high temperature and a seal is complicated; in method (3) the coke powder is remarkable consumed, and the coke powder or the like is apt to be separated from the iron ore due to the difference of specific gravities; and in method (4), complete coating is difficult as the coat increases.
To prevent the sticking in the fluidized bed reduction, there is a method in which iron ore and coal are mixed in the fluidized bed reactor. However, in such method the following problems are expected to take place:
(1) An ash contained in coal should be removed before to introduce into fluidized bed, to reduce gangue content in reduced iron as product. Otherwise the magnetic separation of gangue originated from ash in coal should be required to obtain the desired product.